1. Field of the Invention
The present invention relates to a liquid crystal light modulation element and method of manufacturing same.
2. Description of the Related Art
Conventional liquid crystal light modulation elements are widely known to use a liquid crystal material as a light shutter sandwiched in a space between two substrates. There is known art for maintaining the size of the gap between said substrates in this liquid crystal light modulation element. Such art is known, for example, to provide a seal member of a predetermined thickness along the edges of said substrates, and disperse microparticles of silica or the like as spacers between said substrates.
In recent years the uniformity of the gap between substrates has become more important in conjunction with increasingly high quality and increasing large surface area of the liquid crystal light modulation elements. Conventional liquid crystal light modulation elements are disadvantageous, however, insofar as it is difficult to adequately control the uniformity of the gap between substrates within the light modulation range, such that display irregularities are produced which prevent high quality display from being achieved. Management of this substrate gap is a parameter which determines the display characteristics particularly in liquid crystal light modulation elements using ferrodielectric liquid crystals, and reflective-type liquid crystal light modulation elements using cholesteric (chiral nematic) liquid crystals with circular dichroism. In ferrodielectric liquid crystal light modulation elements, display performance is reduced due to irregular gap between substrates which produces an irregular liquid crystal arrangement. Furthermore, display irregularities become pronounced due to the reduced threshold characteristics when driving elements with a memory such as cholesteric liquid crystal elements.
U.S. Pat. No. 4,249,800 discloses a liquid crystal display device which uses glass beads with an overcoat layer of thermoplastic resin as spacers. This publication gives as an example spacers arranged on a substrate via a so-called photolithography method including photo masking and exposure of a substrate on which has been applied a dispersion of glass beads in a photosensitive lacquer. Another example given pertains to the arrangement of spacers on a substrate via a silk-screening method.
The aforesaid methods include the laborious task of manufacturing glass beads coated with a resin for use as spacers. Since only a small amount of the thermoplastic resin comprising the surface protective coating of the glass beads is in contact with the substrate, adequate adhesion cannot be obtained. It is also difficult to accurately arrange these spacers at optional positions due to the small size of the spacers themselves, thus essentially making it difficult to eliminate display irregularities.
U.S. Pat. No. 5,285,304 discloses a ferrodielectric liquid crystal element crimped between substrates having a dispersion of adhesive type thermosetting particles non-adhesive type thermoplstic particles respectively having a diameter of about 1.5-5 times the thickness of the liquid crystal layer. Japanese Laid-Open Patent Application No. HEI 8-110524 discloses a method wherein a gap controlling material which is not deformable by heat, and a gap controlling material which is deformable by melting or softening then hardened so as to provide adhesive power to both substrates are applied to two substrates, and the latter material is melted or heated above a softening temperature so as to adhere to both substrates.
In the methods of the aforesaid disclosures, however, the polymer particles and gap controlling materials are applied on the substrates, the gap controlling material or polymer particles adhering to the substrates cannot be arranged at desired positions. Therefore, the polymer particles and gap controlling material tend to flocculate during application. As a result, the liquid crystal material is poorly oriented at the locations of said flocculation, which produces an area of inadequate light modulation and prevents obtaining a suitably high quality display.
Japanese Laid-Open Patent Application No. SHO 62-203123 discloses a liquid crystal light modulation element, the object of which is to cut the optional size of the liquid crystal light modulation element using an elongated flexible substrate. This liquid crystal light modulation element is formed using a resin dam of high polymer material in a matrix like continuous coating across the entire surface of a transparent flexible substrate instead of the conventional particle like spacers previously described. This disclosure further states that an adhesion force is generated between said resin dam and said substrates by light pressure. The specific example is given that the resin dam is formed by photolithography using a photoresist, or offset printing of UV hardened resin, and the substrates are bonded by lightly pressing the substrates together at 80.degree. C.
Even when the resin dam is formed as a matrix, for example, it is difficult to realize an accurate gap between substrates only by said resin dam. Furthermore, in the above example, the resin construction material itself is UV hardened resin that must be pressed lightly at about 80.degree. C. to bond the substrates by achieving suitable adhesion. Therefore, a further disadvantage arises in that the application of external pressure and sudden temperature change may produce lumps which increase the gap between the substrates so as to cause improper light modulation by the liquid crystal layer.
Use of a photopolymerizable material to form a resin structural material adhered on top and bottom substrates has been proposed from the perspective of adhesion of the resin structural material. For example, U.S. Pat. No. 5,473,450 discloses a liquid crystal light modulation element formed by a resin wall via photopolymerization phase separation using a photomask. U.S. Pat. No. 5,682,218 discloses a method wherein liquid crystal and unhardened resin are separated by cooling isotropic phase liquid crystal/resin (monomer) mixture, and subsequently hardening the unhardened resin via heat or ultraviolet light to form hardened resin rods.
A disadvantage arises in methods using photopolymerizable materials in that unhardened monomer and polymerization initiator remain in the liquid crystal even after polymerization, thereby affecting the operating characteristics and contrast of the liquid crystal. The resin structural component itself readily produces irregularities that affect the substrate gap.
Japanese Laid-Open Patent No. HEI 9-197412 discloses a liquid crystal light modulation element comprising a light shield area of a liquid crystal display panel provided with a color filter, i.e., an adhesive layer formed of thermoset epoxy resin on a black matrix, formed by silk-screen printing or transfer method, wherein a spacer is fixedly anchored only in said light shield area so as to suppress the poor display qualities caused by flocculation of the spacer.
The resolution of silk-screen printing or transfer methods is nearly 10.times. lower compared to the size of the spacer size so as to cause flocculation of the spacer. Since the adhesive layer is not provided on both substrates, adequate adhesive force cannot be maintained between the substrates. Consequently, a further disadvantage arises in that the application of external pressure and sudden temperature change may increase the gap between the substrates.
Japanese Laid-Open Patent Application No. HEI 9-68698 discloses a liquid crystal display panel having a structure of a plurality of high polymer walls comprised of polyimide in the presence of ether-bonded polyurethane. This liquid crystal display panel provides a chiral nematic liquid crystal encased by macromolecular walls to form a liquid crystal domain.
In this display, however, after the ether-bonded polyurethane is mixed with liquid crystal material and photopolymerizable oligomer, the photopolymerizable oligomer is photopolymerized to form a macromolecular member which completely encases liquid crystal drops, and similar to the previously mentioned art, gives rise to disadvantages such as residual unreacted monomer and resin which prevents attaining a high quality display.
The conventional art described above does not presently provide high quality display without display irregularities and the like, not liquid crystal light modulation element capable of maintaining suitable uniformity of the gap between substrates even when using film substrates or large substrates. Neither does the conventional art provide an element structure of a sufficiently high degree of manufacturability.